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Poisonous Giant Hogweed

Posted by Lorac Tuesday, 12 October 2010

I was travelling a country road and taking some photos in Halton Hills, the area I live in, when I came across this sign. Of course this peeked my interest to look closer!

 I do remember news broadcasts that these were invading Ontario at an alraming rate. To be truthful, I didn't pay a lot of attention. Heracleum mantegazzianum or Asian Poisonous Giant Hogweed, is an invading species. Giant hogweed was first reported in Canada in 1949 and in recent years spread to Southern Ontario.
The concern is it's a very poisonous plant. If you get the sap from this plant on your skin it can cause severe blistering and ad burns. Sap in the eyes can temporarily blind you or it can be permanent! The burns can cause permanent scarring and any areas affected will be super sensitive to sunlight for years to come. It can take up to 48 hours after exposure for the symptoms to appear.

This photo I took below are of dead weeds, but they are still many seeds on them. I couldn't get over the height. These were about 10 feet tall! The plant can grow up to six metres tall, with leaves as big as 1.5 metres across. It is identified by large purple blotches or striping on its stem.

 Below a few of the plants I saw these red markers from the University I think. Not sure if they are marking the seed fall area or new small plant growth.

 It spreads primarily by seeds. Seeds can be carried by vehicles, by people, by winds … it could be a bird. Every plant will produce approimately 500,000 seeds, so the spread potential is huge. This is one of the seed heads I found. You can still see many seeds still attached.


This is a photo below is of what the plant looks like in bloom. I have seen this before and not realized what it was. I thought it was a huge plant and pretty awesome looking and they are quite attractive to look at. I am glad that I didn't stop and take a closer look as I usually would!

Interesting find! Thought I would share it with you.


  1. bonifer Says:
  2. Definitely an interesting post, Lorac, I did hear about this plant on the news, not a fun plant to get too close to!! Thanks for posting!

  3. Barb Says:
  4. Very interesting. Both the flower and the seed looked very much like Cowbane, a CO wildflower.

  5. Unknown Says:
  6. Yes, we have its relative over here, much smaller and to my knowledge not dangerous. We call it shepherd's purse but don't know its botanical name.

  7. Jenn Jilks Says:
  8. There is some Hogweed in Muskoka, too!

    Creepy thing. What were people thinking???

  9. Lyn Says:
  10. I wrote an article about the Giant Hogweed in June. Great shots you have of them but as you said, the plants are highly poisonous! Nice job.

  11. Martha Z Says:
  12. Sounds like a good candidate for eradication.

  13. Deb Says:
  14. We have a neighbour who has at least 6 of these plants. Over the years he has been told about their toxicity but refused to do anything. This summer I noticed they were cut down.

  15. Anonymous Says:
  16. Interesting but also worrying. I hope they find a way to control it.

  17. They are giant! And they look pretty innocuous too - thanks for the post, I had heard about these but like your pictures.

  18. A Scattering Says:
  19. They are hideous and dangerous. A friend of mine found them in her garden - they "lept" the fence from an empty property nearby. The city confirmed what they were, told her how to get rid of them and got rid of them on the empty property. Very proactive effort to get rid of them around here (Woodstock, ON)


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Georgetown, Ontario, Canada
I have lived in Georgetown for 37 years but have traveled around a great deal. I own my own business which takes a lot of my time but try to blog as much as possible! I love to take pictures, no training, just a love of photography. Enjoy the pics but please do not copy them.
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